I have the cree xml as well. Unfortunately, the mount is atrocious. I rigged a quick fix this way:

http://www.bikeforums.net/road-cycli...l#post17363299

I have yet to take the bike out for a test ride with the shimmed up mount, but it's definitely more secure than before.

I know this light is sold with a more secure mount on ebay, but the light is apparently selling so well with the goofy o-ring mount, the manuf. and resellers have little incentive to offer a better option.